Active tectonics of the Loreto area, Baja California Sur, Mexico

Quaternary faulting and coastal uplift characterize the rifted margin of Baja California near Loreto. The process by which strain between the North American and Pacific Plates became partitioned across the Baja Peninsula and the Gulf of California is partially recorded in the tectonics of the Loreto region. The Loreto fault, which consists of a north–south trending northern segment and a northwest–southeast southern segment, records part of this process, as do other faults offshore. During the Pliocene, the southern Loreto fault was actively providing accommodation space for rapid sedimentation in the Loreto basin. Basin subsidence, which abruptly slowed at the end of the Pliocene, was replaced by uplift, which occurred throughout the Quaternary. The Quaternary history of the northern Loreto fault is recorded in fault scarps cutting Quaternary deposits. Fault studies indicate that cumulative normal slip on the northern Loreto fault over the Quaternary was only about 30 m. Latest Pleistocene deposits are offset by about 5 m. Uplifted marine terraces, which may record either isostatic uplift of the coastal region or faulting events offshore, suggest rates of coastal uplift between 0.08 m/ka to 0.16 m/ka for the last 120 ka. These results strongly suggest that since the end of the Pliocene, strain was transferred from the Loreto fault to other faults, probably offshore. Strain between the Pacific and North American plates may, therefore, largely occur in the Gulf of California rather than onshore, in the Loreto region. Despite the low Quaternary strain rates recorded, the offshore faults and the Loreto fault may both represent significant seismic hazards albeit with long recurrence intervals.